From 4490c7c271cb78b3d4adf7d03765f86959cb9192 Mon Sep 17 00:00:00 2001
From: Michael Davis <mcarsondavis@gmail.com>
Date: Sat, 24 Aug 2024 09:43:58 -0400
Subject: [PATCH] Add fast-lanes for '++'/2 and '--'/2 on nil arguments

Currently appending an empty list to another list causes the left-hand
side list to be fully copied. We can skip the append operation when the
right-hand side is nil since it shouldn't change the left-hand side
list. In this fast-lane we scan the list to determine if it is improper
so that cases like `[1, 2 | 3] ++ []` behave the same as cases like
`[1, 2 | 3] ++ [4]`: return a badarg error.

This patch adds similar fast-lanes for '--'/2: we can return the first
argument when either the first or second arguments are nil.
---
 erts/emulator/beam/erl_bif_lists.c | 40 +++++++++++++++++++++++++++++-
 1 file changed, 39 insertions(+), 1 deletion(-)

diff --git a/erts/emulator/beam/erl_bif_lists.c b/erts/emulator/beam/erl_bif_lists.c
index 5dcd53d6139c..08080b713707 100644
--- a/erts/emulator/beam/erl_bif_lists.c
+++ b/erts/emulator/beam/erl_bif_lists.c
@@ -240,6 +240,20 @@ static Eterm append(Export *bif_entry, BIF_ALIST_2) {
         ErtsAppendContext context;
         int res;
 
+        /* Skip appending for `Lhs ++ []`: return `Lhs` instead. */
+        if (is_nil(rhs)) {
+            /* Check that the `lhs` is not an improper list. */
+            Eterm list = lhs;
+            while (is_list(list)) {
+                list = CDR(list_val(list));
+            }
+            if (is_not_nil(list)) {
+                BIF_ERROR(BIF_P, BADARG);
+            }
+
+            return lhs;
+        }
+
         res = append_start(BIF_P, lhs, rhs, &context);
 
         if (res == 0) {
@@ -976,7 +990,31 @@ static int subtract_start(Process *p, Eterm lhs, Eterm rhs,
 static Eterm subtract(Export *bif_entry, BIF_ALIST_2) {
     Eterm lhs = BIF_ARG_1, rhs = BIF_ARG_2;
 
-    if ((is_list(lhs) || is_nil(lhs)) && (is_list(rhs) || is_nil(rhs))) {
+    if (is_nil(lhs) && is_list(rhs)) {
+        /* Fast-lane when the `lhs` is nil: return nil.
+         * First check that `rhs` is not an improper list. */
+        Eterm list = rhs;
+        while (is_list(list)) {
+            list = CDR(list_val(list));
+        }
+        if (is_not_nil(list)) {
+            BIF_ERROR(BIF_P, BADARG);
+        }
+
+        return lhs;
+    } else if (is_list(lhs) && is_nil(rhs)) {
+        /* Similar fast-lane when the rhs is nil: return the lhs list.
+         * First check that `lhs` is not an improper list. */
+        Eterm list = lhs;
+        while (is_list(list)) {
+            list = CDR(list_val(list));
+        }
+        if (is_not_nil(list)) {
+            BIF_ERROR(BIF_P, BADARG);
+        }
+
+        return lhs;
+    } else if ((is_list(lhs) || is_nil(lhs)) && (is_list(rhs) || is_nil(rhs))) {
         /* We start with the context on the stack in the hopes that we won't
          * have to trap. */
         ErtsSubtractContext context;